A system for determining the presence or absence of leakage in a tank system is described in Japanese Patent Application Kokai No. Hei 7-83125. In this system, the pressure in an evaporated fuel discharge prevention system is lowered to a predetermined pressure. The target pressure reduction value of the pressure in the fuel tank is alternately set at an upper-limit value and a lower-limit value, and a feedback pressure reduction process is performed, which gradually causes the pressure in the fuel tank to converge on the target pressure reduction value. The amount of pressure shift in the fuel tank per unit time is calculated (leak down checking mode). In order to eliminate the effect of vapor on the judgment results, the amount of pressure shift per unit time caused by the evaporated fuel is calculated as a correction value. Determination of the presence or absence of leakage in the tank system is accomplished on the basis of a value obtained by subtracting, from the amount of pressure shift calculated in the above-mentioned leak down checking mode, a value produced by multiplying a coefficient by the amount of pressure shift obtained in the correction checking mode. If this value is equal to or less than a predetermined value, it is judged that the tank system is normal, with no leakage. On the other hand, if this value exceeds the predetermined value, it is judged that there is leakage in the tank system.
However, in cases where the leaking hole that is to be detected is a very small hole with a diameter of approximately 0.5 mm, a high degree of precision is required in the diagnostic results indicating the presence or absence of leakage. A disadvantage of the above-described approach is that errors may occur in the diagnostic results in cases where the tank pressure shifts in the negative pressure direction in the correction mode during tank pressure reduction monitoring or prior to tank pressure reduction monitoring.